Gram-positive bacteria are the causative agents of many nosocomial infections and diseases including osteomyelitis and pneumonia. The majority of Gram-positive bacteria harbor sortases, which are transpeptidases that recognize a short peptide sequence found near the C-terminus of proteins. Proteins harboring this recognition motif are subsequently anchored to the peptidoglycan and are required to establish infection by acting as adhesins or by mediating evasion from the host's immune system. One such sortase, SrtA, recognizes and processes the sequence, Leu-Pro-X-Thr-Gly motif in vitro. Bioinformatics analyses predict SrtA may recognize LPXTG variants as well as a 6th amino acid. Another family of sortases, subfamily-3, is also predicted to process similar recognition motifs. We propose to determine the substrate specificity of these sortases to understand in vivo recognition. We will also use NMR spectroscopy to solve the structures of the sortase-substrate complexes. Results from this project will afford details on how these transpeptidases function and will identify residues participating in recognition. This work will complement ongoing sortase inhibitor studies.